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CHAPTER 2

NEW REAGENTS FOR THE SPECTROPHOTOMETRIC

DETERMINATION OF CHROMIUM

2.1 INTRODUCTION

2.2 ANALYTICAL CHEMISTRY

2.3 APPARATUS

2.4 REAGENTS AND SOLUTIONS

2.5 PROCEDURES

2.6 RESULTS AND DISCUSSION

2.7 APPLICATIONS

2.8 CONCLUSIONS

2.9 REFERENCES


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2.1 INTRODUCTION

Chromium was discovered by Nicolas-Louis Vauquelin in 1797 in Siberian

red lead, the mineral crocoite (PbCrO4). Chromium is the sixth most abundant

element in the earths crust. This metal always occurs in combination with other

elements, displaying a wide variety of colors. Fourcroy and Huay suggested the

name chromium (from Greek chroma means color) for this new element because of

its many colored compounds [1]. In 1798, Tobias Lowitz and Martin Heinrich

Klaproth independently found chromium in chromite samples from Russia, and also

Tassaert, a German Chemist at the Paris school of Mines, found it in French

chromite. This ore, a spinel, Fe(CrO2)2, is the only commercial sources of chromium.

Chromium metal was obtained by Moissan in 1893 by reduction of chromic oxide

with carbon in electric furnace. In 1894 Goldschmidt developed the alumino

thermite process for producing chromium by reduction of oxide with aluminium

powder [2]. The only impartant chromium ore is chromite, a spinel [3]. Only

meterorites contain free chromium and most chromium appears in chromite ore,

which contains iron and oxygen [4]. Exposure to chromium compounds occurs

primarily in the occupational setting where chromium commonly is used in the

following 3 basic industries: chemical, metallurgical, and refractory (heat-resistant).

The carcinogenesis of hexavalent chromium compounds was recognized first in the

late 19th century when nasal tumors were described in Scottish chrome pigment

workers [5]. Case reports in the 1930s focused attention on the incidence of lung

cancer in chromate workers, and lung cancer in German chromate workers was

accepted as a work-related disease in 1936 [6].

In the 1960s and 1970s, chromium containing slag was a substantial

component of landfills in a variety of residential and commercial settings in HudsonCounty, New Jersey. Health concerns about exposure to this soil were based on

experience with workers exposed to chromium. Epidemiological studies of chromate

workers indicate an increased risk of death from lung cancer in workers exposed to

hexavalent chromium compounds [7,8]. Hexavalent chromium compounds are both

skin and pulmonary sensitizers, producing a generalized irritation of the conjunctiva

and mucous membranes, nasal perforations [9], and a contact dermatitis [10]

(Blackjack disease in card players exposed to chromium in green felt). Trivalent


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chromium is an essential trace metal necessary for the formation of glucose

tolerance factor and for the metabolism of insulin.

The two most important function of chromium in steels are improving the

mechanical properties particularly hardenability and increasing the corrosion

resistance [11]. The magnitude of the effect in each case is roughly proportional to

the percent chromium in steel. Low chromium steels (< 3 % of Cr) produced in all

structural shapes such as bars, tubes, sheets, plates etc., are used extensively as

engineering materials in every branch of industry. For all but heaviest duty

applications, chromium content is generally less than 6%. Steels contain more than

10% chromium and are designated stainless because of their resistance to corrosion

and oxidation. Non-hardenable grade contain 0.08-0.2 % carbon and 11.25-27.0%

chromium. Type 430 (AISI) is used in large quantities for trim on buildings,

automobiles, etc., and for nitric acid manufacturing equipment. The austenitic

stainless steels (non-hardenable) contain 16-26% of chromium and 0.15-1.25%

carbon.

Chromium is the recently recognized biologically essential trace metal. The

first conclusive evidence demonstrating a metabolic role of chromium was obtained

by Mertz and Schwarz in a series of investigation of which the first report appeard in

1955 [12,13].

Trivalent chromium is an essential trace metal necessary for the normal

metabolism of cholesterol, fat and glucose. Although trivalent chromium has the

potential to form complexes with proteins and nucleic acids, hexavalent chromium

must first be converted to the trivalent state before it combines with nucleic acids

and proteins [14]. Trivalent chromium forms tight bonds with oxygen and sulfur

containing ligands and some chromium complexes with histidine and cysteine are

relatively inert because of these tight bonds [15]. Trivalent chromium potentiates the

action of insulin, probably through a glutathione-like complex composed of niacin,

trivalent chromium and amino acids [16].

Chromium deficiencies in the diet produce elevated circulating insulin

concentrations, hyperglycemia, hypercholesterolemia, elevated body fat, decreased

sperm counts, reduced fertility, and a shortened life span. Severe chromium


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deficiency may cause weight loss, poor coordination, destruction of the nerves in the

extremities of the body and inflammation of the brain. The effects of excessive

dietary chromium are not well known. Some forms of chromium that are found in

the environment may be cancer causing (carcinogenic), but this type of chromium is

different from dietary chromium. Food with high chromium content are fruits,

vegetables, whole grains (oats and barley) seeds, nuts, legumes (peas and beans) and

brewers yeast. When these food is processed, particularly using stainless steel

equipment (e. g. when cocking or canning), their chromium content may increase.

Allergy to chromium compounds carries in men a worse prognosis than dose

sensitization to other allergens. The reason is not known. Continued contact with

unrecognized chromium compounds in the environment or possibly ingestion of

chromium compounds have been considerd as possible explanation [17].

The determination of micro amounts of chromium in soil and other naturally

occurring materials are of considerable interest because of the contrasting biological

effects of its two common oxidation states, chromium(III) and chromium(VI).

Chromium(III) is an essential nutrient for maintaining normal physiological function

[18] whereas, chromium(VI) is toxic [19]. It is difficult, however, to determine

chromium directly in natural water samples because of its very low concentration

level. It is known that an increase in the content in soils makes them infertile and

toxic effect depends to some extent on the chromium oxidation state. On the other

hand the introduction of chromium salts in to soil have some positive effects due to

activation of some biochemiocal processes [20].

2.2 ANALYTICAL CHEMISTRY

Many methods have been reported for the quantitative determination of

chromium. The analytical technique varies from intensively coupled plasma-atomic

emission spectroscopy [21], neutron activation analysis [22], X-ray absorption

spectroscopy [23], atomic absorption spectroscopy [24], complexometry [25],

catalytic-kinetic [26], flow injection [27], sequential injection [28,29] to flourogenic

method [30]. Divrikli et al. [31] developed a method for the determination of

chromium based on co-precipitation with cerium(IV) hydroxide by flame atomic

absorption spectrometry.


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A survey of literature revealed that a large number of reagents are suitable

for the spectrophotometric determination of chromium. Cahnmann and Ruth [32]

used 1,5-diphenylcarbazide as a spectrophotometric reagent for the determination of

chromium in blood, in the course of investigations of lung cancers among chromium

workers by using a Beckman DU spectrophotometer and a wave length of 543 nm,

the reaction was sensitive to 0.005 ppm.

Miller and Yoe [33] used diphenylcarbazide as a spectrophotometric reagent

for the determination of chromium in human plasma and red cells. Traces of

chromium in human blood was determined by a method which utilized the red-violet

complex formed by the reaction of Cr2O72- with diphenylcarbazide. The

concentration of chromium in normal human plasma ranged from 0.017 to 0.052

ppm, which was in good agreement with previously reported values. The

concentration of chromium in red cells ranged from 0.014-0.038 ppm. Standard

deviation by the method was 1.6%. Motojima and Hashitani [34] reported 8-

hydroxyquinaldine as a spectrophotometric reagent for the determination of

chromium in uranium. Todorovska et al. [35] described the direct electrothermal

atomic absorption spectrometry (ETAAS) for the determination of chromium in

serum and urine samples without any preliminary sample pretreatment. Rosa and

Maria [36] determined the total amount of chromium based on an on-line

ultrasound-assisted sample digestion procedure exploiting the stopped-flow mode,

followed by flow injection chromium preconcentration using a minicolumn filled

with a commercially available chelating resin (Chelite Che).

Hoshi et al. [37] proposed a method for the spectrophotometric

determination of chromium(VI) based on preconcentration with collection of metal

complexes on a chitin has been applied to the spectrophotometric determination of

chromium(VI) in water. The chromium(VI) was collected as its 1,5-

diphenylcarbazide -(DPC) complex on a column of chitin in the presence of dodecyl

sulfate as counter-ion. The Cr-DPC complex retained on the chitin was eluted with a

methanol-1 M acetic acid mixture (7:3) and the absorbance of the eluent was

measured at 541 nm. Revanasiddappa and Kiran Kumar [38] used leuco xylene

cyanol-FF as a sensitive reagent for the spectrophotometric determination of trace

amounts of chromium in steels, industrial effluents and pharmaceutical samples.


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The method was based on the oxidation of leuco xylene cyanol-FF to its blue form

of xylene cyanol-FF by Cr(VI) in H2SO4 medium (pH 1.2-2.4), the absorbance of

the formed dye was measured in an acetate buffer medium (pH 3.0-4.6) at 615 nm.

The method was obeyed Beers law in the concentration range of 0.05-mL-1

of chromium. Molar absorptivity and Sandells sensitivity of the system was

8.23x104 L mol-1cm-1-2

Narayana and Cherian [39] used variamine blue as a chromogenic reagent for

the spectrophotometric determination of trace amounts of chromium.

Chromium(VI) reacts with potassium iodide in acid medium to liberate iodine,

which oxidizes variamine blue to form a violet colored species having an absorption

maximum 556 nm. Beers law was obeyed in the range 2-12 gmL -1 of Cr(VI).

Same authors used azure B [40] as a chromogenic reagent for the

spectrophotometric determination of trace amounts of chromium. The method was

based on the oxidation of azure B. Molar absorptivity and Sandells sensitivity of the

system were found 3.77x104 L mol-1cm-1 and 2.76x10-2 gcm-2 respectively. Cheng

[41] used xylenol orange and methylthymol blue as chromogenic reagents for the

spectrophotometric determination of trace amounts of chromium. The molar

absorptivity was found to be 19.0 x 103

L mol-1

cm-1

. Mohamed and El-Shahat [42]

developed a method for the spectrophotometric determination of Cr(VI) based on

their reactions with perphenazine to instantaneously give a red colored product

exhibiting a maximum absorbance at 526 nm. Girish Kumar and Muthuselvi [43]

used 2-hydroxybenzaldiminoglycine as a reagent for the spectrophotometric

determination of trace amounts of chromium.

Zaitoun [44] described a method for the spectrophotometric determination of

chromium(VI), based on the absorbance of its complex with 1,4,8,11-

tetraazacyclotetra-decane (cyclam). The complex showed a molar absorptivity of 1.5

104

L mol-1

cm-1

at 379 nm. Under optimum experimental conditions, a pH of

4.5 and 1.960 103

mgL-1

cyclam were selected, and all measurements were

performed 10 min after mixing. Major cations and anions did not showed any

interference; Beer's law was obeyed in the concentration range 0.2-20 mgL-1

with a

detection limit of 0.001 mgL-1

. The standard deviation in the determination is 0.5


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mg L-1 for a 15.0 mg L-1 solution. Carvalho et al. [45] used 4-(2-thiazolylazo)-

resorcinol as a reagent for the spectrophotometric determination of chromium.

Boef and Poeder [46] described spectrophotometric determinations of

milligram amounts of chromium(III) with complexans, based on the fact that the

chromium(IIl) complexes are formed rapidly at boiling temperatures, but very

slowly at room temperature, while the formation of some interfering complexes

takes place instantaneously. Determinations with EDTA were more sensitive, but the

combined presence of cobalt and other metals still interferes; there was no

interference with the less sensitive NTA. The combined presence of a l00-fold

amount of copper, nickel, cobalt and iron generally has no effect on the results. The

use of DCTA, DTPA and HEDTA was discussed. Katsuya and Yukiteru [47] used

2-hydroxy-1-(2-hydroxy-4-sulfo-1-naphtyhylazo)-naphthalein, Jacobsen and Lund

[48] used 3-thianaphthenoyltrifluoro-acetone and Johnston and Holland [49] used

thioglycollic acid for the spectrophotometric determination of chromium.

Yotsuyanagi et al. [50] described the extraction-spectrophotometric

determination of chromium(III) with 4-(2-pyridylazo)-resorcinol (PAR). PAR(H2R)

forms a 1:3 complex with chromium(III) in a boiling acetate buffer solution at pH 5.

The complex forms an ion-association compound with tetradecyldimethylbenzyl

ammonium ion (TDBA+):Cr(R)(HR)2

--TDBA

+which can be extracted into

chloroform, the molar absorptivity being 4.7x104

at 540 nm. If EDTA was added as

a masking agent after the Cr(HR)3 has been formed, only iron, cobalt and nickel

interfere seriously, and the method can be made specific for chromium by a

preliminary extraction of these metals with cupferron. The sensitivity of the method

was seven times higher than that of the diphenylcarbazide method. Gowda and Raj

[51] reported fluphenazine hydrochloride as a reagent for the determination of

chromium. Fluphenazine hydrochloride formed a red colored species with

instantaneously at room temperature in 2.0- 5.5 M H3PO4 medium. The red colored

species exhibited maximum absorbance at 500 nm with molar absorptivity of

2.061x 104 L mol-1cm-1. Beers law was valid over the concentration range 0.05-1.85

ppm of chromium. Rizvi et al. [52] used tropolone for the spectrophotometric

determination of chromium. Chromium formed a golden yellow complex with


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tropolone on heating on a water bath, the colored moiety was extracted in CHCl3.

The complex exhibited maximum absorbance at 400 nm.

Hussain and Venkata [53] used cyclohexane-1,3-dionebisthiosemicarbazione

monohydrochloride for the rapid spectrophotometric determination of chromium(VI).

The reagent produced yellow colored solutions with Cr(VI) in NaOAC-HCl medium.

Molar absorptivity value of the system was 1.21x104

L mol-1

cm-1

at 370 nm.

Nagaraj et al. [54] developed a spectrophotometric method for the determination of

chromium. The method was based on the diazotization of dapsone in hydroxylamine

hydrochloride medium and coupling with N-(1-napthyl)ethylenediamine

dihydrochloride by electrophilic substitution to produce an intense pink azo-dye,

which has absorption maximum at 540 nm. The Beer's law was obeyed from

0.02--1

and the molar absorptivity was 3.4854x104

L mol-1

cm-1

. The limits

-1 and

-1

respectively. Ram et al. [55] used malachite green for the

spectrophotometric determination of chromium in waste water. Reagent formed a

green colored complex with chromium in acetic acid at pH 2.5. The Molar

absorptivity of the system was found to be 8.0x104

L mol-1

cm-1

at 560 nm.

Benzyltributylamonium [56] was also reported as spectrophotometric reagent for the

determination of chromium in waste water and steels.

Fabiyi et al. [57] reported, variamine blue as a chromogenic reagent for the

spectrophotometric determination of nano amounts of chromium. Chromium(VI)

reacts with potassium iodide in acid medium to liberate iodine that oxidizes

variamine blue to produce violet-colored substances having an absorption maximum

at 615 nm with molar absorptivity of 8.12x104

L mol-1

cm-1

. Beers law was valid

over the concentration range 0.0003-15 -1

. Kamburova [58] described for the

spectrophotometric determination of chromium. The interaction of

iodonitrotetrazolium chloride and tetrazolium violet with chromium(VI) has been

studied and the formation of ion-associates with a 1:1 composition in hydrochloric

acid medium established. Extraction photometric methods for determination of

chromium in steels and soils have been developed. Same author used methylene blue

[59] as reagent for the determination of chromium. The interaction of Cr(VI) and the

thiazine dye methylene blue has been examined. The ion-associate formed is


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extractable into 1,2-dichlorethane. Triphenyltetrazolium chloride [60] used by the

same author for the spectrophotometric determination of chromium.

Yoshiaki et al. [61] described a method for the determination of chromium.

Chromium(VI) reacts with O,O-dibutyl dithiophosphate ion to form tris(dibutyl

dithiophosphato)chromium(III) and tetrabutyl thiophosphoryl disulfide in an acidic

solution (pH 1.2-1.7). Both of the products are extracted into hexane and the

absorbance at 278 nm based on these products was utilized for the determination of

chromium(VI). The chromium(III) complex corresponding to two-thirds of the

initial chromium(VI) concentration and the disulfide corresponding to three-halves

of the initial chromium(VI) concentration were extracted under the optimum

conditions. The calibration graph obtained is linear over the range of

210-6

-110-4

mol/dm3

chromium(VI) concentration, and the apparent molar

absorptivity was 1.69104

mol-1

dm3

cm-1

. Arya and Anvita [62] used ferron for the

spectrophotometric determination of chromium.

Raj and Gowda [63] described a method for the spectrophotometric

determination of chromium by using thioridazine hydrochloride. The reagent forms

a blue colored radical cation with chromium(VI) instantaneously at room

temperature in 14 mol L1

orthophosphoric acid medium. The blue species exhibits

an absorption maximum at 640 nm with a molar absorption coefficient of

2.577104

L mol1

cm1

. Rosa et al. [64] proposed a method for the

spectrophotometric determination of chromium with 2-(5-chloro-2-pyridylazo)-5-

dimethylaminophenol. A useful absorptionmetric method was proposed for Cr(III)

in concentrations ranging from 15 to 400 ppb. The methods were applied to

chromium determination in water samples with very satisfactory results. Maheswari

and Balasubramanian [65] used Rhodamine-6G, Ressalan et al. [66] used

3-hydroxy-3-phenyl-1-o-hydroxyphenyltriazene and Ressalan et al. [67] used

3-hydroxy-3-phenyl-p-tolyl-1-o-nitrophenyltriazene for the determination of

chromium.

Melwanki and Seetharamappa [68] reported propericiazine as a

spectrophotometric reagent for the determination of chromium in environmental

samples. Propericiazine formed a red colored radical cation, exhibited maximum

absorption at 510 nm in H3PO4 medium. Beers law was valid over the concentration


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range of 0.15-2.25 mgL-1. The Sandells sensitivity of the reaction was found to be

3.42 ngcm-2

. Methdilazine hydrochloride [69] was also reported as reagent for the

spectrophotometric determination of chromium. Revanasiddappa and Kiran Kumar

[70] used trifluroperazine hydrochloride as a reagent for the spectrophotometric

determination of chromium. The method was based on the oxidation of

trifluroperazine hydrochloride by chromium(VI) in the presence of H3PO4. The red

colored species exhibited an absorption maximum at 505 nm. The system was

obeyed Beers law at 2-18 g of chromium(VI) in a final volume of 10 mL. The

molar absorptivity of the color system was 2.08x104 L mol-1 cm-1 and the developed

color was stable for 2 hours.

Lourenco et al. [71] reported a spectrophotometric method for the

determination of chromium, which was based on the formation of

chromium(III)/azide complexes was established by investigating a new band in the

ultraviolet region. The maximum molar absorptivity coefficient at 287 nm

(1.4810.008104 L mol-1 cm-1), leading to the determination of metal ion

concentrations one hundred times lower than the ones formerly determined in the

visible region. The system obeyed Beers law and is suitable for chromium

determination in the 0.702-2.81 mgL-1

concentration range. Revanasiddappa and

Kiran Kumar [72] reported citrazinic acid as a new coupling agent for the

spectrophotometric determination of trace amounts of chromium by oxidation of

hydroxylamine in acetate buffer of pH 4.00.5. Molar absorptivity of the system was

2.12x104

L mol1

cm1

and Sandells sensitivity of the system was 0.00246 gcm-2

at

470 nm. The color was stable for 6 hours and the system was obeyed Beers law in

the range 2.0-15 g of Cr(VI) in a final volume of 10 mL.

Cherian and Narayana [73] reported saccharin as a new coupling agents for

the spectrophotometric determination of chromium. The method obeyed Beers law

in the concentration range of 1-16 gmL1

for chromium with p-nitroaniline-

saccharin and 0.6-14 gmL1 of chromium with sulphanilamide-saccharin couples.

The molar absorptivity, Sandells sensitivity of the systems with p-nitroaniline-

saccharin and sulphanilamide-saccharin couples were found to be

5.41104

L mol1

cm1

, 1.93103

gcm2

and 2.63104

L mol1

cm1

, 3.9103

gcm2

, respectively.


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Tarafder et al., [74] reported an improved spectrophotometric method for

chromium(VI) determination in rocks, minerals and water samples. The method was

based on the determination of nitrite produced by Cr(VI) during the oxidation of

hydroxylamine (NH2OH). Beer's law was obeyed in the range of 0.02-10 gmL-1

of

Cr(VI). The molar absorptivity and Sandell's sensitivity of the system were found to

be 4.2x104 L mol-1 cm-1 -2, at 540 nm Comparison of

spectrophotometric reported methods for the determination of chromium(VI) with

proposed method are summarized in table 2.1

The aim of the present work is to provide a simple and sensitive method for

the determination of chromium using -naphthol,-naphthol, pyrocatechol and N-

(1-naphthyl)ethylenediamine dihydrochloride as new coupling agents. The proposed

method has been employed to the determination of chromium in alloy, soil sample

pharmaceutical preparation and natural water samples.

2.3 APPARATUS

2.3.1 Spectrophotometer

A SHIMADZU (Model No: UV-2550) UV-Visible spectrophotometer with

1 cm matching quartz cells were used for the absorbance measurements.

2.3.2 pH Meter

A WTW pH 330 pH meter was used.

2.4 REAGENTS AND SOLUTIONS

All chemicals used were of analytical reagent grade, and doubly distilled

water was used in the preparation of all solutions in the experiments. Standard stock

solution containing 1000 gmL

-1

of chromium(VI) was prepared by dissolving0.2829 g of K2Cr2O7 in 100 mL volumetric flask with distilled water and

standardized by titrimetric method [25]. The stock solution was further diluted as

needed. 2,4-Dinitrophenylhydrazine (DNPH)(1%), -naphthol (1%), -naphthol

(1%), pyrocatechol (1%) and N-(1-naphthyl)ethylenediamine dihydrochloride

(NEDA)(1%) were used. The following reagents were prepared by dissolving

appropriate amounts of reagents in distilled water: 2M HCl and 2M NaOH.


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2.5 PROCEDURE

2.5.1 Using 2,4-Dinitrophenylhydrazine--Naphthol as Reagents

Aliquots of sample solution containing 0.02-4.0 gmL-1

of chromium(VI)

was transferred in to series of 10 mL calibrated flask. To each of the flask, 1 mL of

2,4-dinitrophenylhydrazine and 1 mL of 2 M HCl solutions were added. Each

-naphthol and

1 mL of 2M NaOH solution, again allowed to stand for 5 min with occasional

shaking to complete the reaction. After dilution to 10 mL with distilled water, the

absorbance of the red colored dye was measured at 663 nm against the

corresponding reagent blank and the calibration graph was constructed. The results

are summarized in table 2.2A

2.5.2 Using 2,4-Dinitrophenylhydrazine- -Naphthol as Reagents


of chromium(VI)


2M HCl and 1 mL of 2,4-dinitrophenylhydrazine solution were added. Each mixture

-naphthol and 1 mL of

2M NaOH, again allowed to stand for 5 min with occasional shaking to complete the

reaction. After dilution to 10 mL with distilled water, the absorbance of the violet

colored dye was measured at 503 nm against the corresponding reagent blank and

the calibration graph was constructed. The results are summarized in table 2.2B

2.5.3 Using 2,4-Dinitrophenylhydrazine- Pyrocatechol as Reagents


of chromium(VI) was

transferred in to 10 mL calibrated flask. To each of the flask, 1 mL of 2 M HCl and

1 mL of 2,4-dinitrophenylhydrazine solution were added. Each mixture was allowed

to stand for 5 min, and then added 1 mL of 1% pyrocatechol and 1 mL of 2M

NaOH, again allowed to stand for 5 min with occasional shaking to complete the

reaction. After dilution to 10 mL with distilled water, the absorbance of the violet

colored dye was measured at 619 nm against the corresponding reagent blank and

the calibration graph was constructed. The results are summarized in table 2.2C.


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2.5.4 Using 2,4-Dinitrophenylhydrazine- N-(1-Naphthyl)ethylenediamine

Dihydrochloride as Reagents


of chromium(VI)


2,4-dinitrophenylhydrazine and 1 mL of 2 M HCl solutions were added. Each

mixture was allowed to stand for 5 min, then added 1 mL of 1% N-(1-

naphthyl)ethylenediamine dihydrochloride and 1 mL of 2M NaOH solution, again

allowed to stand for 5 min with occasional shaking to complete the reaction. After

dilution to 10 mL with distilled water, the absorbance of the red colored dye was

measured at 525 nm against the corresponding reagent blank and the calibration

graph was constructed. The results are summarized in table 2.2D

2.5.5 Analysis of Chromium in Alloy Steel

About 0.1 g of a steel sample containing 1.02% of chromium was weighed

accurately and placed in a 50 mL beaker. To it, added 10 mL of 20% H2SO4 and

carefully covered with a watch glass until the brisk reaction subsided. The solution

was heated and simmered gently after addition of 5 mL of conc. HNO3 until all

carbides were decomposed. Then, 2 mL of a 1:1 H2SO4 solution was added and the

mixture was evaporated carefully until the dense white fumes derived off the oxides

of nitrogen, and then cooled to room temperature. After appropriate dilution with

water, the contents of the beaker were warmed to dissolve the soluble salts. The

solution was then cooled and neutralized with a dilute NH4OH solution. The

resulting solution was filtered. The residue (silica) was washed with a small volume

of hot 1% H2SO4 followed by water and the volume was made up to the mark with

water. Suitable aliquots of sample solution were analyzed according to the procedure

for chromium. The results are summarized in table 2.3A, 2.3B, 2.3C and 2.3D.

2.5.6 Determination of Chromium in Soil

An air-dried homogenized soil sample (1 g) was weighed accurately and

placed in a 100 mL Kjeldahl flask. The sample was digested and the content of flask

was filtered through a Whatman no. 40 filter paper into a 25 mL calibrated flask and

neutralized with dilute ammonia. It was then diluted to the mark with water.

Appropriate aliquots of 1-2 mL of the solution was transferred in to a 10 mL

calibrated flask and analyzed for chromium content according to the general


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procedure. They tested negative. To these samples known amounts of chromium(VI)

sample was added and analyzed by the proposed procedure for chromium. The

results are summarized in table 2.2A, 2.2B, 2.2C and 2.2D.

2.5.7 Determination of Chromium in Water Samples

Each filtered environmental water sample (100 mL) was analyzed for

chromium. They tested negative. To these samples known amounts of

chromium(VI) sample was added and analyzed by the proposed procedure for

chromium.

2.5.8 Analysis of Chromium in Pharmaceutical Preparation

Samples of the finely ground multivitamin/multimineral tablet (Chromoplex)containing chromium was treated with 5 mL 2M HNO3, and the mixture was

evaporated to dryness. The residue was leached with 5 mL 0.5 M H 2SO4. The

solution was diluted to a known volume with water, after neutralizing with dilute

ammonia. Suitable aliquots of sample solution were analyzed by the present method

for chromium determination. The results are summarized in table 2.4A, 2.4B, 2.4C

and 2.4D.

2.6 RESULTS AND DISCUSSION

2.6.1 Absorption Spectra

2.6.1.1 -naphthol as a reagent

This method is based on the oxidation of 2,4-dinitrophenylhydrazine and

coupling reaction. Chromium(VI) oxidizes 2,4-dinitrophenylhydrazine to its

diazonium salt in an acid medium. The diazonium salt is then coupled with

-naphthol in an alkaline medium, which gives an azo dye with absorption

maximum at 663 nm. Diazotization and coupling reactions are found to be

temperature dependent. Diazotization is carried out at 0-50C and coupling reaction is

carried out at room temperature, above 350C there is a decrease in intensity of the

color. The absorption spectrum of the colored species of an azo dye is presented in

figure II.1A and reaction system is presented in scheme 2.1.

2.6.1.2 Using-naphthol as a reagent




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-naphthol in an alkaline medium, which gives an azo dye with absorption



carried out at room temperature, above 350

there is a decrease in intensity of the

color. The absorption spectrum of the colored species of an azo dye is presented in

figure II.1A and reaction system is presented in scheme 2.1.

2.6.1.3 Using as N-(1-naphthyl)ethylenediamine dihydrochloride as a reagent



diazonium salt in an acid medium. The diazonium salt is then coupled with N-(1-

naphthyl)ethylenediamine dihydrochloride in an alkaline medium, which gives an

azo dye with absorption maximum at 525 nm. Diazotization and coupling reactions

are found to be temperature dependent. Diazotization is carried out at 0-50C and

coupling reaction is carried out at room temperature, above 350 there is a decrease in

intensity of the color. The absorption spectrum of the colored species of an azo dye

is presented in figure II.1B and reaction system is presented in scheme 2.1.

2.6.1.4 Using pyrocatechol as a reagent




pyrocatechol in an alkaline medium, which gives an azo dye with absorption



carried out at room temperature, above 350C there is a decrease in intensity of the

color. The absorption spectrum of the colored species of azo dye is presented in

figure II.1B and reaction system is presented in scheme 2.1.

2.6.2 Effect of Reagent Concentration

A volume of 1 mL of 1% 2,4-dinitrophenylhydrazine solution was required

for maximum absorbance and it was found that, addition of 1 mL-naphthol

(1%) or 1 mL-naphthol (1%) or 1 mL of NEDA (1%) or 1 mL of pyrocatechol

(1%) reagents provides maximum absorbance. Larger excess of reagent produced no


16/37

further increase in the absorbance. Oxidation of 2,4-dinitrophenylhydrazine by

chromium is most effective in acidic medium. The reaction is studied using excess

2,4-dinitrophenylhydrazine and varying quantities of HCl. The reaction is allowed to

-naphthol (1%)

-naphthol (1%) or NEDA (1%) or pyrocatechol (1%) in an alkaline medium.

Absorbance of the azo dyestuff is then measured at 663 nm or 503 nm or 525 nm or

619 nm against reagent blank. It was found that absorbance at 663 nm or 503 nm or

525 nm or 619 nm was maximum. When concentration of HCl during the oxidation

reaction is greater than 2M and further increase in acid concentration does not affect

the absorbance.

2.6.3 Analytical Data

2.6.3.1 Using -naphthol as a reagent

In this method adherence to Beers law is studied by measuring the

absorbance values of solutions varying chromium concentration. A straight line

graph is obtained by plotting absorbance against concentration of chromium. Beers

law is obeyed in the concentration range 0.024.0 gmL of chromium. Adherence

to Beers law graph for the determination of chromium using -naphthol is presented

in figure II.2A. The molar absorptivity and Sandells sensitivity of the method are

found to be 1.2x104

L mol-1

cm-1

and 4.3x10-3

gcm. The detection limit

(DLL=1

the regent blank (n=5) and S is the slope of the calibration curve) of the chromium

determination are found to be 0.752 gmLand 2.280 gmLrespectively.

2.6.3.2 Using -naphthol as a reagent




law is obeyed in the concentration range 0.059.0 gmL

of chromium. Adherence

to Beers law graph for the determination of chromium using -naphthol is presented

in figure II.2B. The molar absorptivity and Sandells sensitivity of the method are

found to be 3.44x104

L mol-1

cm-1

and 1.51x10-3

gcm. The detection limit

(DLL


17/37

the regent blank (n=5) and S is the slope of the calibration curve) of the chromium

determination are found to be 0.616 gmLand 1.866 gmLrespectively.

2.6.3.3 Using pyrocatechol as a reagent




law is obeyed in the concentration range 0.16.0 gmL of chromium. Adherence

to Beers law graph for the determination of chromium using pyrocatechol is

presented in figure II.2C. The molar absorptivity and Sandells sensitivity of the

method are found to be 1.96x104 L mol-1 cm-1 and 2.6x10-3 gcm. The detection

limit (DL=3.3 L=10

deviation of the regent blank (n=5) and S is the slope of the calibration curve) of

the chromium determination are found to be 0.332 gmL

and 1.008 gmL

respectively.

2.6.3.4 Using N-(1-naphthyl)ethylenediamine dihydrochloride as a reagent




law is obeyed in the concentration range 0.014.0 gmL

of chromium. Adherence

to Beers law graph for the determination of chromium using N-(1-

naphthyl)ethylenediamine dihydrochloride is presented in figure II.2D. The molar

absorptivity and Sandells sensitivity of the method is found to be 1.56x104

L mol-1

cm-1

and 3.3x10-3

gcm, The detection limit (DL

limit (QL

S is the slope of the calibration curve) of the chromium determination are found to

be 0.252 gmL

and 0.765 gmL

respectively.

2.6.4 Effect of Divers Ions

The effect of various non-target species on the determination of chromium

was investigated. The tolerance limits of interfering species are established at those

concentrations that do not cause more than 2% error in absorbance values with

fixed concentration of chromium. The present method is based on the oxidation of

DNPH with chromium then coupled with coupling reagents. Therefore, strong


18/37

oxidizing or reducing species are expected to interfere. The results indicated that

Ce(IV) and Hg(II) showed severe interference. However, the tolerance level of these

ions may be increased by the addition of 2 mL of 2% EDTA. The results are given

in table 2.5A, 2.5B, 2.5C and 2.5D.

2.7 APPLICATIONS

The developed method is applied to the quantitative determinations of traces

of chromium in different samples such as alloys, pharmaceutical preparations,

natural water and soil samples. Statistical analysis of the results by t- and F- tests

show that, there is no significant difference in accuracy and precision of the

proposed and reference method [57]. The precision of the proposed method is

evaluated by replicate analysis of samples containing chromium at different

concentration.

2.8 CONCLUSIONS

1. The reagents provide a simple and sensitive method for the

spectrophotometric determination of chromium.

2. The reagents have the advantage of high sensitivity and low absorbance of

reagent blank.

3. The developed method does not involve any stringent reaction conditions

and offers the advantages of color stability over 5 hours.

4. The statistical analysis of the results by t- and F- tests show that, there is no

significant difference in accuracy and precision between the proposed

method and reference method.

5. The proposed method successfully applied for the determination of

chromium(VI) in pharmaceutical, steel and environmental samples.


19/37

TABLE 2.1: COMPARISON OF SPECTROPHOTOMETRIC METHODS FOR

THE DETERMINATION OF CHROMIUM(VI) WITH PROPOSED METHOD

ReagentMolar

Absorptivity

(L mol-1

cm-1

)

Remarks

1. Variamine blue [57] 8.12 x 103

Low sensitive

2. Citrazinic Acid [72] 2.12 x 104

Less stable less detection limit

3. Chitin [37] 3.50 x 104

Require solvents for the extraction

4. Trifluoperazine-

hydrochloride [70]

2.08 x 104 Color is stable only 2 h and less

detection limit

5. Cyclam [44] 1.50 x 104 Low detection limit not applicable to

lower concentration (0.220 mgL-1

)

6. 4-(2-Thiazolylazo)-

resorcinol [45]

2.73 x 104 N-Cetyl-,N,Ntrimethylammonium

bromide is required for the reaction

7. Perphenazine [42] 1.87 x 104

Color is stable up to 30 min only

Proposed Reagents

DNPH --naphthol

DNPH --naphthol

1.20 x 104

3.44 x 104

Color is stable up to 5 hours, less

interference, facile, sensitive, rapid

and non extractive


20/37

TABLE 2.2A: DETERMINATION OF CHROMIUM IN NATURAL WATER

AND SOIL SAMPLE USING DNPH--NAPHTHOL REAGENTS

Proposed method Reference method [57]

Sample

Cr(VI)

added

-1

Cr(VI)

found in

-1SDa

Recovery

%

Cr(VI)

found in

-1 SDa

Recovery

%

t-testb F-testc

Natural

water

1.0

2.0

3.0

0.96 0.05

1.96 0.05

2.95 0.06

96.00

98.00

98.33

0.95 0.05

1.97 0.06

2.97 0.06

95.00

98.50

99.00

1.78

1.78

1.78

1.04

1.04

1.04

Soil

1.0

2.0

3.0

0.99 0.04

1.99 0.04

2.98 0.06

99.00

99.50

99.33

0.99 0.04

1.99 0.04

2.98 0.04

99.00

99.50

99.33

0.60

0.51

0.78

1.16

1. 09

2.03

aMeanStandard deviation (n = 5)

bTabulated t-value for 8 degrees of freedom at P(0.95) is 2.65

c

Tabulated F-value for (4, 4) degrees of freedom at P (0.95) is 5.72.


21/37

TABLE 2.2B: DETERMINATION OF CHROMIUM IN NATURAL WATER

AND SOIL SAMPLE USING DNPH --NAPHTHOL REAGENTS


Sample

Cr(VI)

added

-1

Cr(VI)

found in

-1 SDa

Recovery

%

Cr(VI)

found in

-1 SDa

Recovery

%

t-testb F-testc

Natural

water

1.0

2.0

3.0

0.99 0.025

1.98 0.028

2.99 0.036

99.00

99.00

99.66

0.95 0.05

1.97 0.06

2.97 0.06

95.00

98.50

99.00

1.78

1.78

1.78

1.04

1.04

1.04

Soil

1.0

2.0

3.0

0.98 0.028

1.99 0.030

2.99 0.035

98.00

99.50

99.66

0.99 0.04

1.99 0.04

2.98 0.04

99.00

99.50

99.33

0.60

0.51

0.78

1.16

1. 09

2.03



c Tabulated F-value for (4, 4) degrees of freedom at P (0.95) is 5.72.


22/37

TABLE 2.2C: DETERMINATION OF CHROMIUM IN NATURAL WATER

AND SOIL SAMPLE USING DNPH- PYROCATECHOL REAGENTS


Sample

Cr(VI)

added

-1

Cr(VI)

found in

-1SDa

Recovery

%

Cr(VI)

found in

-1 SDa

Recovery

%

t-testb F-testc

Natural

water

1.0

2.0

3.0

1.02 0.02

2.01 0.04

2.99 0.03

102.00

100.50

99.66

0.95 0.05

1.97 0.06

2.97 0.06

95.00

98.50

99.00

2.23

0.27

0.79

2.32

2.25

1.77

Soil

1.0

2.0

3.0

0.99 0.03

1.98 0.02

2.98 0.03

99.00

99.00

99.33

0.99 0.04

1.99 0.04

2.98 0.04

99.00

99.50

99.33

0.79

2.23

1.49

1.77

2.23

2.10





23/37

TABLE 2.2D: DETERMINATION OF CHROMIUM IN NATURAL WATER

AND SOIL SAMPLE USING DNPH- NEDA REAGENTS


Sample

Cr(VI)

added

-1

Cr(VI)

found in

-1SDa

Recovery

%

Cr(VI)

found in

-1 SDa

Recovery

%

t-testb F-testc

Natural

water

1.0

2.0

3.0

0.99 0.02

1.99 0.02

2.99 0.03

99.00

99.50

99.66

0.95 0.05

1.97 0.06

2.97 0.06

95.00

98.50

99.00

1.06

1.01

0.79

1.53

1.39

1.36

Soil

1.0

2.0

3.0

0.98 0.02

2.01 0.03

2.99 0.04

98.00

100.5

99.66

0.99 0.04

1.99 0.04

2.98 0.04

99.00

99.50

99.33

2.23

0.65

0.57

2.04

1. 37

1.69





24/37

TABLE 2.3A: DETERMINATION OF CHROMIUM IN ALLOY USING DNPH -

-NAPHTHOL REAGENTS

Sample

Chromium

certified

in %

Amount of

chromium

found SDa

Recovery

%t-test

bF-test

c

GKW Steel, India

(0.05g/100mL); C 0.54,

Mn 0.89, S 0.018, P

0.034, Si 0.33, V 0.13

1.02 1.01 0.04 99.01 0.57 1.05


b Tabulated t-value for 4 degrees of freedom at P(0.95) is 2.78

cTabulated F- value for (4,4) degree of freedom at 95% probability level is 6.39;

TABLE 2.3B: DETERMINATION OF CHROMIUM IN ALLOY USING DNPH -

-NAPHTHOL REAGENTS

Sample

Chromium

certified in

%

Amount of

chromium

found SDa

Recovery

%t-test

bF-test

c

GKW Steel, India

(0.05g/100mL); C 0.54,

Mn 0.89, S 0.018, P

0.034, Si 0.33, V 0.13

1.02 1.01 0.034 99.01 0.65 1.00





25/37

TABLE 2.3C: DETERMINATION OF CHROMIUM IN ALLOY USING DNPH-

PYROCATECHOL REAGENTS

Sample

Chromium

certified

in %

Amount of

chromium

found SDa

Recovery%

t-testb

F-testc

GKW Steel, India

(0.05g/100mL); C 0.54,

Mn 0.89, S 0.018, P

0.034, Si 0.33, V 0.13

1.02 1.01 0.03 99.01 0.29 2.25

a MeanStandard deviation (n = 5)b

Tabulated t-value for 4 degrees of freedom at P(0.95) is 2.78


TABLE 2.3D: DETERMINATION OF CHROMIUM IN ALLOY USING DNPH-

NEDA REAGENTS

Sample

Chromium

certified

in %

Amount of

chromium

found SDa

Recovery

%t-test

bF-test

c

GKW Steel, India

(0.05g/100mL); C 0.54,

Mn 0.89, S 0.018, P

0.034, Si 0.33, V 0.13

1.02 1.01 0.03 99.01 0.82 1.82


b Tabulated t-value for 4 degrees of freedom at P(0.95) is 2.78



26/37

TABLE 2.4A: DETERMINATION OF CHROMIUM IN PHAMACEUTICAL

PREPARATION USING DNPH- -NAPHTHOL REAGENTS

SampleChromium

certified

Chromium

found SDa

Recovery

%t-test

bF-test

c

Chromoplex 0.200 0.198 0.02 99.00 1.86 1.17

aMeanStandard deviation (n = 5) [mg/tablet]



TABLE 2.4B: DETERMINATION OF CHROMIUM IN PHAMACEUTICAL

PREPARATION USING DNPH - -NAPHTHOL REAGENTS

SampleChromium

certified

Chromium

found SDa

Recovery

%t-test

bF-test

c

Chromoplex 0.200 0.198 0.03 99.00 1.78 1.04


bTabulated t-value for 4 degrees of freedom at P (0.95) is 2.78



27/37

TABLE 2.4C: DETERMINATION OF CHROMIUM IN PHAMACEUTICAL

PREPARATION USING DNPH-PYROCATECHOL REAGENTS

Sample Chromium

certified

Chromium

found SDa

Recovery

%

t-testb F-testc

Chromoplex 0.200 0.199 0.03 99.50 0.82 2.19




TABLE 2.4D: DETERMINATION OF CHROMIUM IN PHAMACEUTICAL

PREPARATION USING DNPH- NEDA REAGENTS

Sample Chromium

certified

Chromium

found SDa

Recovery

%

t-testb

F-testc

Chromoplex 0.200 0.200 0.04 100.00 0.55 1.00





28/37

TABLE 2.5A: EFFECT OF DIVERSE IONS ON THE DETERMINATION OF

CHROMIUM (2 gmL) USING DNPH- -NAPHTHOL REAGENTS

Diverseions

Tolerance

limit

(gmL)

Diverseions

Tolerancelimit(gmL)

Diverseions

Tolerance

limit

(gmL)

Na+

CHCOO

K+

BO33

Mg2+

Ca

2+

Mn2+

Ni2+

3000

3000

3000

3000

200

200200

200

citrate

oxalate

tartarate

Al3+

Cd2+

Ba

2+

Co2+

Zn2+

500

500

500

500

500

500500

500

Cu2+

*

Ce4+*

Fe3+

*

Sn2+

*

Pb2+*

Hg

2+

*W

6+*

Mo6+

*

25

25

25

25

25

2525

25

* Masked by masking agent

TABLE 2.5B: EFFECT OF DIVERSE IONS ON THE DETERMINATION OF

CHROMIUM (3 gmL) USING DNPH - -NAPHTHOL REAGENTS

Diverse

ions

Tolerance

limit(gmL

)

Diverse

ions

Tolerance

limit(gmL

)

Diverse

ions

Tolerance

limit(gmL

)

K+

BO33

Na+

CHCOO

tartarate

citrate

oxalate

Ba2+

2500

2500

2500

2500

2500

2500

2500

500

Co2+

Zn2+

Al3+

Cd2+

Mn2+

Ni2+

Mg2+

Ca2+

500

500

100

100

100

100

100

100

Sn2+*

Pb2+

*

W6+

*

Mo6+

*

Hg2+*

Cu2+

*

Ce4+

*

Fe3+

*

50

50

50

50

50

50

50

50



29/37

TABLE 2.5C: EFFECT OF DIVERSE IONS ON THE DETERMINATION OF

CHROMIUM (3 gmL) USING DNPH-PYROCATECHOL REAGENTS

Diverse

ions

Tolerance

Limit(gmL)

Diverse

ions

Tolerance

limit(gmL)

Diverse

ions

Tolerance

limit(gmL)

Na+

K+

CHCOO

Citrate

Oxalate

Tartarate

BO33

Al3+

> 3000

> 3000

> 3000

> 3000

> 3000

2500

1000500

Cd2+

Ba2+

Co2+

Zn2+

Mg2+

Ca2+

Mn

2+

Ni2+

500

500

500

500

300

300

300300

Sn2+

*

Pb2+

*

Fe3+

*

W6+

*

Mo6+

*

Cu2+

*

Ce

4+

*Hg

2+*

50

50

50

50

50

20

2050


TABLE 2.5D: EFFECT OF DIVERSE IONS ON THE DETERMINATION OF

CHROMIUM (2 gmL) USING DNPH-NEDA REAGENTS

Diverse

ions

Tolerance

limit(gmL

)

Diverse

ions

Tolerance

limit(gmL

)

Diverse

ions

Tolerance

limit(gmL

)

Na+

K+

CHCOO

citrate

oxalate

tartarate

BO33

> 3000

> 3000

> 3000

> 3000

> 3000

> 3000

2000

SO42

Al3+

Cd2+

Ba2+

Co2+

Zn2+

Mg2+

1000

800

800

800

500

500

200

Ca2+

Mn2+

Ni2+

Cu2+

*

Ce

4+

*Hg

2+*

200

200

200

25

2525



30/37

FIGURE II.1A: ABSORPTION SPECTRA OF AZO DYES: DNPH-

-NAPHTHOL COUPLE VS REAGENT BLANK (a), DNPH- -NAPHTHOL

COUPLE VS REAGENT BLANK (b) AND REAGENT BLANK VS DISTILLED

WATER (c)

FIGURE II.1B: ABSORPTION SPECTRA OF AZO DYES: DNPH- NEDA VS

REAGENT BLANK (a), DNPH- PYROCATECHOL COUPLE VS REAGENT

BLANK (b) AND VS REAGENT BLANK VS DISTILLED WATER (c)


31/37

FIGURE II.2A: ADHERENCE TO BEERS LAW FOR THE DETERMINATION

OF CHROMIUM USING DNPH--NAPHTHOL REAGENTS

FIGURE II.2B: ADHERENCE TO BEERS LAW FOR THE DETERMINATION

OF CHROMIUM USING DNPH--NAPHTHOL REAGENTS


32/37

FIGURE II.2C: ADHERENCE TO BEERS LAW FOR THE DETERMINATION

OF CHROMIUM USING DNPH- PYROCATECHOL REAGENTS

FIGURE II.2D: ADHERENCE TO BEERS LAW FOR THE DETERMINATION

OF CHROMIUM USING DNPH- NEDA REAGENTS


33/37

REACTION SCHEMES 2.1

O2NNH

NH 2

NO 2

+ Cr2O 72- O2N N

+

N

NO 2

+ Cr(III) + H 2O

O2NN

+

N

NO 2

+

OHOH -

O2N

N

N

NO 2

OH

O2NN

+

N

NO 2OH O2N

N

N

NO 2

OH

+

OH -

ClHCl

-

Cl-

Cl-

O 2N N+

N

NO 2

+NH

NH 2

O H -

O2N N+

N

NO 2OH

OH

O2N

N

N

NO 2

OH

OH+

O H -

O 2N

N

N

NO 2

NH

NH 2

Cl-

Cl-

2HCl


34/37

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